Endogenous neutrophil elastase was shown to cleave cell-bound C3b nd C3bi, two fragments of C3 which play a major role in phagocytosis. When viable PMN were incubated with sheep cells bearing 125I-C3bi, release of rediolabel occurred immediately, resulting in release of red cells from the PMN. C3b cleavage was slower and EC3b remained bound up to 30 min. Release was blocked with normal human serum whereas serum from patients with alpha 1 antitrypsin deficiency had little inhibitory effect. This suggests a mechanism for the in vivo regulation of PMN proteases. The expression of Complement Receptors (CR) by Raji cells and other Burkitt's lymphoma cell lines was studied in rosette assays and in competitive inhibition assays with purified C3 and C4 fragments. Raji cells expressed the three described C3 receptors, CR1, CR2, and CR3. However, the affinity of Raji cell CR1 was very low when compared with normal B lymphocytes and Raji Cells failed to bind the C4b, aligand which binds to CR1 on normal cells. The significance of this difference with regard to receptor-mediated cell function is now under study.